In a two-cycle engine because each cylinder fires on every cycle instead of on every second cycle, a two-stroke engine should, in theory, be capable of developing twice the horsepower of a conventional four-stroke engine having the same volumetric displacement. In practice, this is not the case, the main reason being that, in any present two-stroke carbureted engines, there is no provision to completely separate the spent exhaust gases from the incoming, fuel-charged, intake air. This means that, to prevent unburnt gas from being lost with the exhaust gas, the valving must be arranged such that some spent exhaust gases remain in the cylinder. This results in a lower power output than would otherwise be expected.
Another major problem with conventional two-stroke engines is that, because the crank case is used as a pre-compression chamber, the lubricating oil must be mixed with the gasoline and is burnt along with the fuel. As well, in order to ensure that sufficient lubrication is available to coat the cylinder walls, an oil/fuel mixture is required wherein the ratio of oil to fuel is much higher than is normally consumed in a comparably-sized four-stroke engine. The result is the well-known smoky, dirty, high-emission, two-stroke engine.
In a multi-cylinder engine, one of the reasons that the crankshaft has to be relatively large is that the high thrust forces exerted on the crankshaft by the piston of the cylinder undergoing combustion must be transmitted as a torque through the crankshaft and thence to the adjacent piston, or pistons which are undergoing intake, compression or exhaust strokes, as the case may be. Any residual torque produced over and above that required by the adjacent cylinders is available as useable power. But because of the necessary requirement to continually transmit power to the adjacent cylinders from the one undergoing combustion, the crankshaft has to be made sufficiently large and durable to handle these large torque loads. Due to the constraints imposed by materials, the bearing surfaces supporting the crankshaft, as well as those journals used to connect the connecting rods to the crankshaft, have to be so large that sliding friction bearing surfaces rather than ball or roller bearings, are generally used at all journal bearing points.
In addition to the complexities caused by having to transmit torque loads to the adjacent cylinders as each cylinder fires in turn, the crankshaft also has to be configured so as to accommodate the selected firing order. Further, the crankshaft usually incorporates integral counterweights for dynamic balancing of the pistons. On top of all this, the crankshaft--at least in 4-stroke engines--usually also incorporates lubrication channels which deliver oil to all of the bearing journals as well as to the lower cylinder walls. In meeting all of the required crankshaft durability and functional requirements, this results in an engine component that requires complicated manufacturing processes and expensive toolage with a high resulting cost of manufacture.
Another inherent deficiency in any crankshaft-based method for converting the reciprocating motion of the pistons into rotary motion of the crankshaft, is that a significant portion of the combustion gas forces acting on the head of the piston end up as high side forces acting between the piston and cylinder walls. This is due to the fact that the connecting rod is at an angle relative to the piston line of travel during the time that the greatest combustion forces are applied to the piston. These high side forces acting principally during combustion, but also during the other strokes, do no useful work and end up as frictional heat - which adds to the problem of cooling. These high side forces can be reduced, to some extent, by making the connecting rod longer, reducing the maximum angle of deflection; however, this approach causes other problems, thus the connecting rod is usually made as short as possible in most automotive engines due to size limitations.
In automotive engines where overall size is a major restriction and wherein the connecting rods are therefore made as short as possible, special provisions must be made in the piston design to accommodate these high side forces. To deal with the problem of high piston-to-cylinder side forces pistons are usually fabricated with an integral skirt at the bottom which serves to provide an extended piston bearing surface against the cylinder. Furthermore, the lower portion of the piston, including the skirt, is usually made slightly elliptical to accommodate wear. But both of these provisions add to the complexity of the piston over what would be required if straight back-and-forth motion, only, was required. Another problem that must be dealt with in crankshaft-based internal combustion engine design is that of piston `slap`. Piston `slap`, as it is sometimes called, is that additional side force acting on the piston due to the fact that the lower, or crankshaft, portion of the connecting rod is moving in a circular path while the piston end is moving in a straight back-and-forth path. This means that the center of mass of the connecting rod transcribes an elliptical orbit and induces an additional side force on the piston proportional to the engine RPM. These forces also tend to cause a severe bending moment in the connecting rods and, as a result, designers go to great lengths to make the connecting rods as light and as strong as possible. But these necessary provisions also add cost to the overall engine.
Additionally, in a crankshaft-based engine, some provision must be made to deliver lubricant to the piston wrist pin or it would quickly overheat and seize up. In most four-stroke automotive engines, provision is made to direct oil from the crankshaft, through the connecting rod, and thence to the piston wrist pin. But this, too, adds complexity and cost to the conventional automotive engine.
In conventional two-stroke engines, the partially-compressed fuel and oil charged air flows into the cylinder at the same time, or very closely following, the discharge of the spent exhaust gases. Unlike the situation in a four-stroke engine, which has very well defined intake, compression, power and exhaust strokes, a two-stroke engine attempts to achieve all of this in just two strokes. This results in some inevitable mixing of the fuel and oil charged intake gases with the spent exhaust gases. If the valve ports are designed such that the exhaust port is uncovered by the piston on the downstroke well in advance of the intake port being uncovered, then most of the spent exhaust gases will be discharged before the fuel and oil charged air enters the cylinder. But on the subsequent piston up stroke, the exhaust port will remain uncovered too long and some unspent fuel and oil charged air will be lost to exhaust. The converse is that the exhaust valve may be designed to open at the same time as, or slightly after, the exhaust port is uncovered, in which case too much spent exhaust gas will remain in the cylinder and will result in less than optimum power output.
Compression ignition or diesel cycle engines, because of the much higher compression levels required in order to effect combustion, necessarily have to utilize much stronger and heavier pistons, connecting rods, crankshaft and cylinders than are required in comparable spark ignition engines having a similar power output. As well, because of the higher levels of heat generated in a compression ignition engine, a larger and more sophisticated cooling system must be used. The result is that the typical diesel engine is invariably significantly heavier and more costly than a comparably sized spark ignition engine.
White in U.S. Pat. No. 4,608,951 and Kurek et al in U.S. Pat. No. 4,803,964 both employ means for conversion of reciprocating to rotary motion without the use of a crankshaft; however, both of these patents employ a pinion gear which tracks along an elongated ring gear and does not completely remove piston `slap` and, as well, would not be suitably durable. In addition, the design is not easily adapted to use in a two-cylinder, horizontally- opposed configuration and thus is not really suitable. Rucker in U.S. Pat. No. 5,233,949, Koderman in U.S. Pat. No. 3,886,805 and Wickman in U.S. Pat. No. 3,693,464 all describe a type of epicyclic gear crank method for direct conversion of reciprocating to rotary motion. The above noted Patents however involve complex arrangements of bearings and gears which are time consuming and complex to assemble.
Many current larger-size agricultural and industrial diesel engines utilise a turbine to recover power from the exhaust gases that would otherwise be lost. In some instances the power recovery turbine is directly connected to a rotary compressor which acts as a supercharger to provide a boost in compression level in the diesel engine itself.
In some respects, a diesel engine which utilises a power recovery turbine to direct-drive a rotary compressor or supercharger is somewhat like a gas turbine engine wherein the combustor or combustion chamber is replaced by the diesel engine to generate heat. However, in such a diesel engine, all of the output power is derived from the diesel portion of the engine, with the power recovery turbine providing supplementary power to drive the supercharger only.